The invention relates broadly to a redundant data processing system. In particular, such a data processing system as based on programmed logic controllers (PLC's) must meet stringent requirements with respect to availability, for being able to control sophisticated industrial and similar systems, inasmuch as failure of the overall system would often be a fatal incident. Under certain circumstances a few minor functions may fail, although anyway, the overall system should remain `on`. Tandemized data processing systems have been contemplated for some time, but the present invention intends to fill a need for such systems that are flexible, and moreover, may change between various operative conditions through external control as well through internally detecting various types of hazardous conditions.
More in particular, the invention relates to a tandemized data processing system comprising a first and a second programmed logic controller and centralized governance means for alternatively governing one controller in a master state, whilst allowing the other controller in standby state, each controller being provided with a local powering facility and a local bus, and each local bus being attached via a respective bus interface element to a general bus provided with input/output subsystems. Now, although such system provides for automatic interchanging between the two controllers, the inventor has recognized that also adequate and immediate operator interaction with the master controller should be possible at all times.